Time-overcurrent relays



Feb. 3, 1959 E. PADDISON TIME-OVERCURRENT RELAYS Filed Jan 7, 1957 FIGJbUnited States PatentO TIME-OVERCURRENT RELAYS Eric Paddison, Stalford,England, assignor to The English Electric Company Limited, London,England, a British company Application January 7, 1957, Serial No.632,845

Claims priority, application Great Britain January 11, 1956 5 Claims.(Cl. 317-167) This invention relates to electrical protection relays ofthe time-overcurrent kind which operate on the in duction principle.Such relays have a drive magnet which is energized in accordance withthe current in a protectedcircuit and an induction armature which isarranged to move in an air gap in the drive magnet.

In such a relay the operating torque applied to the induction armaturevaries in approximate direct proportion to the square of the currentenergising the magnet and by arranging for suitable damping means to actupon the induction armature the relay can be caused to have an inversetime-overcurrent operating characteristic.

There are applications in which it is desirable to have a relayoperating characteristic which is very inverse. For the purposes of thisspecification the meaning of a "very inverse relay operatingcharacteristic is one which approaches closely to a curve having theform I t=constant, where I is the operating current and t is time. Forsome applications a very inverse operating characteristic is ideal.However, in practice there are several causes which are particularlyimportant on low and high operating currents and, which prevent such acharacteristic from being obtained and it is desirable to provide in atime-overcurrent relay some simple means by which the form of thecharacteristic can be modified. The present invention relates to a relaywhich incorporates means for eifecting one form of such a modification.

According to the invention, in an electrical time-overcurrent relay ofthe kind comprising an electroconductiv'e' armature, a shaded pole driveelectro-magnet adapted to urge said armature in a relay operatingdirection by electro-magnetic induction when energised by an operatingcurrent, and permanent-magnetic damping means acting on said armature,said drive electro-magnet has a readily-saturable magnetic reluctancepath in series with a main air gap in which said armature moves, and alessreadily-saturable magnetic reluctance path directly shunting saidmain air gap.

According to a feature of the invention, said lessreadily-saturablemagnetic reluctance path includes an air gap.

According to a preferred feature of the invention, in an electricaltime-overcurrent relay of the kind comprising an electroconductivearmature, a shaded pole drive electro-magnet adapted to urge saidarmature in a relay operating direction by electro-magnetic inductionwhen energised by an operating current, and'permanent-magnetic dampingmeans acting on said armature, said drive electro-magnet has asubsidiary air gap in series with a main air gap in which said armaturemoves, with a saturable magnetic bridge member accross said subsidiaryair gap and a further saturable magnetic bridge member across said mainair gap, the latter bridge member being designed to saturate at a highermagnet-energising current than the first-mentioned bridge member.

According to a further feature of the invention, an element having themagnetic character of an air gap 2,872,626 Patented Feb. 3, 1959described with reference to the accompanying drawing a bridge electricalcontacts in which Fig. la shows a front elevation view and Fig. 11')shows a plan view of a time-overcurrent relay.

Referring now to the drawing, a time-overcurrent relay is shown toinclude anelectro-magnet 10 which is arranged to be energised by amagnetising coil 11. An induction armature in the form of a metal disc12 is mounted so as to turn about its centre on a spindle 13 and isbiased towards a neutral position by the spring 14. An arm 15 carried bythe spindle is arranged to 16 when the disc 12 moves a predetermineddistance against the bias of the spring 14. Damping means in the form ofa permanent magnet 17 are also provided as shown in Fig. la and Fig. lb.

An electro-magnet 10 has a main air gap 10:! in which the disc 12 moves.There is another air gap 101: which is bridged by a saturable member10c, this member having a very high permeability and therefore beingeasily saturated. The coil 11 carries the operating current and producesthe magnetic flux through the magnet 10, some of this flux linking thedisc 12. Shading rings 10a cause the torque to be exerted on the disc inthe contact closing direction. Thus, when the current through the coil11 is small, the saturable member 100 being unsaturated, the operatingflux produced by this current will be relatively greater than it is whenthe current is large and the member 100 is saturated. There is,therefore, a tendency for the relay to operate relatively faster onsmall operating currents due to the provision of this 7 saturable member10c.

The electro-magnet 10 also has a member 10d across the main air gap 10a.However, this bridge member 10d is separated from the electro-magnet 10by two non-magnetic spacers 10 This arrangement is such that at mediumovercurrents this bridge member 10a is not saturated and thereforeserves to divert a considerable proportion of the flux set up by thecoil 11 from the working air gap 10a- The proportion diverted issubstantially constant up to a particular value of current but forcurrents exceeding this the bridge member 10d saturates. This saturationlimits the amount of flux diverted from the main air gap 10a and resultsin a relative increase of the operating torque applied to the disc 12.Again, this serves to decrease the operating time of the relay, but thistime on high overcurrents.

The result is that on low and high overcurrents, the operating time ofthe relay is reduced but on medium overcurrents it remains substantiallyunaffected.

The advantage of this will readily be understood by considering theeifects of the control spring 14, friction at the bearings of thespindle 13, the effects of the inertia of the disc 12, the dynamicbraking effect of the driving electro-magnet 10 and the naturalsaturation tendency of the electro-magnet 10, when the operatingcurrents are very high.

Although a very inverse characteristic may be expected from aconsideration of the simple theory of an induction drive the aboveeffects contribute to cause the operating characteristic to deviate fromthis.

If no control spring is used and if the damping magnet produces aconstant flux the damping force will be directly proportional to thespeed of the disc, but even if the torque from the drive magnet isexactly proportional to the square of the operating current the timecharacteristic would still not be inversely proportional to the squareof the current over a wide current range.

saturable bridge The reasons for this are, firstly that at low currentsthe driving torque is small and more close in magnitude to thefrictional torque which is practically constant. Friction has relativelyno importance at high currents but its effect is to increase theoperating time of the relay at low currents. Secondly, although at lowcurrents, where the operating time is relatively long, the inertiaeffect of the disc is negligible, at high overcurrents Where theoperating time is short the inertia of the disc becomes important andserves to delay the operation of the relay. Thirdly, on highovercurrents a dynamic damping effect is caused by the driveelectro-magnet and whereas the normal damping action of the permanentmagnet would lead to an ideal operating characteristic the dynamicdamping action has the effect of increasing the operating time of therelay on high overcurrents. The effect of saturation of theelectro-magnet on high overcurrents modifies the characteristic stillfurther and it will be seen from these considerations that these variousfactors serve to increase the operating time of the relay on low andhigh overcurrents but not on medium currents. By utilising the inventiona compensating effect can, therefore, be achieved since the function ofthe saturable bridge members utilised by the invention is to decreasethe operating time of the relay, on low and high overcurrents but not onmedium currents.

Whereas, in theory, it may seem possible to achieve the same technicalresult by using separate saturable magnetic devices in the energisingcircuit of the drive electromagnet as is achieved by the saturablefeatures provided by the invention, the construction according to theinvention is preferred for the following reasons. Firstly, the inventionprovides a cheap and simple means for introducing saturable featuresand, secondly, the performance of the relay is determined solely by thesignal applied to the relay energising coil 11. If independentmagnetically saturable devices were incorporated in the circuitof thiscoil, the performance of the relay would to some extent depend upon thenature of the power source energising the relay. For example, the natureof the operating characteristic of the relay would depend upon whetheror not the relay was energised from a current transformer or a voltagetransformer. This feature is of less consequence if the relay has a formaccording to the invention.

In its preferred form the invention provides two magnetically saturablefeatures on a single drive magnet.

One of these features serves to modify the relay characteristic on lowoperating currents and the other feature serves to modify the relaycharacteristic on high operating currents. 7

What I claim as my invention and desire to secure by Letters Patent is:

1. An electrical time-.overcurrent relay of the kind comprising anelectroconductive armature, a shaded pole drive electro-magnet adaptedto urge saidtarmature in a relay operating direction by electro-magneticinduction when energised by an operating current, and permanentmagneticdamping means acting on said armature, said drive electro-magnet havinga readily-saturable magnetic reluctance path in series with .a main airgap in which said armature moves, and a less-readily-saturable mag neticreluctance path directly shunting said main air gap.

2. An electrical time-overcurrent relay of the kind comprising anelectroconductive armature, a shaded pole drive electro-magnet adaptedto urge said armaturein a relay operating direction by electro-magneticinduction when energised by an operating current and permanentmagneticdamping means acting on said armature, said drive electro-rnagnet havinga subsidiary air gap in series with a main air gap in which saidarmature moves, with a saturable magnetic bridge member across saidsubsidiary air gap and a further saturable magnetic bridge member acrosssaid main air gap, the latter bridge member being designed to saturateat a higher magnet-energising current than the first-mentioned bridgemember.

3. An electrical time-overcurrent relay according to claim 1 whereinsaid less-readily-saturable magnetic reluctance path includes an airgap.

4. An electrical time-overcurrent relay according to claim 2 wherein anelement having the magnetic character of an air gap is included in themagnetic circuit of said further saturable magnetic bridge member.

5. An electrical time-overcurrent relay according to I claim 4 whereinsaid element having the magnetic char- References Cited in the file ofthis patent UNITED STATES PATENTS Smith July 30, 1918 Goldsborough July17, 1928

